1. Field of the Invention
The present invention relates to a support structure carrying a thrust load of a transmission, a method of manufacturing the support structure and a thrust needle roller bearing.
2. Description of the Background Art
A thrust needle roller bearing is constituted of needle rollers, a cage and a race, and is structured to allow the needle rollers to be in line contact with the race. Therefore, despite of a small bearing area, this type of bearing is advantageous because of its high load capacity and high stiffness. The thrust needle roller bearing is thus suitable for use under hostile conditions, for example, during a drive under conditions like scarce lubrication and high-velocity rotation, and is accordingly used as a support structure carrying a thrust load of an automatic transmission of an automobile (e.g. support structure for a continuously variable transmission).
A conventional thrust needle roller bearing is known from Japanese Patent Laying-Open No. 2002-70872 according to which at least one of the inflow and the outflow of a lubricating oil is improved or promoted so as to increase the amount of the lubricating oil which passes through the bearing per unit time. This thrust needle roller bearing is described below in connection with FIGS. 17A-17C.
As shown in FIGS. 17A-17C, this thrust needle roller bearing 50 has a plurality of needle rollers 80 and two annular cages 60 and 70. These two cages 60 and 70 respectively have a plurality of windows 61 and 71 having a length in the radial direction longer than that of the rollers. Roller holder portions 64 and 74 formed at these windows 61 and 71 hold needle rollers 80 therebetween from above and below. The radial length la of roller holder portions 64 and 74 of two cages 60 and 70 is made smaller than the length l of the rollers. At least one of two cages 60 and 70 is bent so that at least one of the thicknesses t1 and t2 is smaller than the thickness t0 in the direction perpendicular to the radial direction as seen in FIG. 13B (hereinafter referred to as perpendicular direction). Here, thicknesses t1 and t2 refer to respective thicknesses of portions of cages 60 and 70 that are located respectively outside and inside in the radial direction with respect to roller holder portions 64 and 74, and the thickness t0 refers to the thickness of roller holder portions 64 and 74 in the perpendicular direction.
Outer plate-like portions 62 and 72 of two cages 60 and 70 are laid on each other in the perpendicular direction while the innermost parts 67 and 77 respectively of inner plate-like portions 63 and 73 are bent in the perpendicular direction to be laid on each other. The innermost part 67 of inner plate-like portion 63 is caulked and thereby fixed.
In this way, on at least one of radially outer portions 62 and 72 and radially inner portions 63 and 73 that are smaller in thickness than roller holder portions 64 and 74, the inflow or outflow of the lubricating oil can be improved or promoted to increase the amount of the lubricating oil passing through the bearing per unit time. Moreover, since the passage of the lubricating oil is less prone to be blocked by cages 60 and 70, the lubricating oil does not stay in the bearing. Thus, any increase of the oil temperature can be avoided and the durability of the bearing can be improved.
It has been done in some cases that car manufactures and automatic-transmission manufactures use an oil to which an additive is added for energy saving. The additive-containing oil is inferior to normal oils in lubricating ability for bearings. When such an oil is used for a conventional single-row thrust roller bearing as shown in FIGS. 17A-17C, the following problems occur.
The conventional thrust needle roller bearing 50 shown in FIGS. 17A-17C is structured to allow needle rollers 80 to be in line contact with the race and thus the raceway surface is in rolling line contact with needle rollers 80. Accordingly, the circumferential velocity is higher at a radially outer part of the raceway surface relative to the circumferential velocity at the center of rotation of the bearing. Thrust needle rollers in a single row are usually used in a portion carrying a thrust load of such a transmission as CVT (Continuously variable transmission). Then, there is a difference in circumferential velocity between the raceway surface and the roller. A maximum difference in circumferential velocity arises, between the difference in circumferential velocity between the raceway surface and the roller at the inner end of the roller and the difference in circumferential velocity therebetween at the outer end of the roller. This difference is greater as the roller is longer with respect to the outer diameter of the roller, resulting in a greater differential slip (skew of the roller). The occurrence of the differential slip causes breakage of an oil film and consequently metal-to-metal contact. Then, the metal-to-metal contact portion generates heat and thereby surface damage (smearing) as well as surface-originated peeling are likely to occur, particularly when the rotational speed is higher. In addition, it is often seen that the lifetime becomes shorter due to the above factors.
Some thrust needle roller bearings are structured so that the cage and the race are in sliding contact with each other. In such a case, the cage blocks flow of a lubricating oil to resist the flow thereof to an area where needle rollers and the race are in rolling contact with each other. In particular, it is necessary that the oil flows to the rolling surface of the needle rollers. If the amount of the oil is scarce, metal-to-metal contact between the needle rollers and the race occurs, possibly resulting in surface damage at an earlier stage.
Box-shaped cages 60 and 70 of the conventional thrust needle roller bearing 50 block flow of the lubricant to needle rollers 80 and thus the surface damage at an earlier stage mentioned above is likely to occur. Further, although two cages 60 and 70 are attached to each other and the periphery thereof is caulked, this manner of caulking could cause two cages 60 and 70 to separate from each other.